Frequency hopping receivers typically include a mixer selectively coupled to one of a plurality of local oscillators (LOs) producing LO signals cycling at different frequencies to down convert a frequency-moving RF signal. For instance, in a two-LO frequency hopping receiver, at a particular time the first LO is coupled to the mixer to down convert the received RF signal, while the second LO is de-coupled from the mixer. At another particular time, the first LO is de-coupled from the mixer, while the second LO is coupled to the mixer to down convert the received RF signal. To reduce interference in the down conversion of the received RF signal, it is desirable that there be no leakage of the LO signal generated by the unselected LO to the mixer. This concept is further explained in more detail with reference to an exemplary prior art receiver.
FIG. 1 illustrates a block diagram of a prior art frequency-hopping receiver 100. The prior art receiver 100 consists of a low noise amplifier (LNA) 102, an image-reject filter 103, a mixer 104, an intermediate frequency (IF) filter 106, and an IF amplifier 108. In addition, the prior art receiver 100 consists of an LO circuit having first LO source 110, a second LO source 112, a first set of switching devices 114 and 115 in the form of field effect transistors (FETs), a second set of switching devices 116 and 117 also in the form of FETs, and a pair of 50-ohm loads.
The LNA 102 amplifies the received RF signal. The image-reject filter 103 further reject (i.e. suppresses) the image signal with respect to the desired signal. The mixer 104 mixes the RF signal with an LO signal generated by either one of the LO sources 110 (which generate LO signals cycling at different frequencies) to down convert the received RF signal to an IF signal. The IF filter 106 removes undesirable signals from the IF signal. And, the IF amplifier 108 amplifies the IF signal.
In receiving a frequency-hopping RF signal, the first and second sets of switches 114–115 and 116–117 are operated to alternate the coupling of the LO sources 110 and 112 to the mixer 104. More specifically, at a particular time the FET 114 is turned on to couple the first LO source 110 to the mixer 104 and the FET 115 is turned off to de-couple the LO source 110 from the 50-ohm load. At the same time, the FET 116 is turned off to de-couple the second LO source 112 from the mixer 104 and the FET 117 is turned on to couple the LO source 112 to the 50-ohm load. At another time, the FET 114 is turned off to de-couple the first LO source 110 from the mixer 104 and the FET 115 is turned on to couple the LO Source 110 to the 50-ohm load. At the same time, the FET 116 is turned on to couple the second LO source 112 to the mixer 104 and the FET 117 is turned off to de-couple the LO source 112 from the 50-ohm load.
A problem with the prior art receiver 100 is that the first and second sets of FETs 114–115 and 116–117 are not perfect in isolating the unselected LO source from the mixer. That is, when the FET 114 is turned on and FET 115 is turned off in order to couple the first LO source 110 to the mixer 104, and the FET 116 is turned off and the FET 117 is turned on in order to de-couple the second LO source 112 from the mixer 104, there is still some leakage of the second LO signal across the FET 116. Conversely, when the FET 114 is turned off and FET 115 is turned on in order to de-couple the first LO source 110 from the mixer 104, and the FET 116 is turned on and FET 117 is turned off in order to couple the second LO source 112 to the mixer 104, again there is some leakage of the first LO signal across the FET 114. This leakage LO signal combines with the desired LO signal at the input to the mixer 104, and thus causes interference in the down conversion of the received RF signal.
Thus, there is a need to improve the isolation between the unselected LO source and the mixer. Such need and others are met with an improved LO circuit in accordance with the invention.